The proposed project will continue an ongoing, normative, longitudinal study of bone homeostasis in white and black women, two racial groups with differing fracture prevalence. Its purpose is to investigate the mechanisms by which black women may be protected from osteoporosis and to delineate the natural history of bone loss and its determinant in a healthy, biracial female population. While it is generally believed that black women have a lower incidence of osteoporotic fracture than whites, the reason for their apparent protection is unknown. Although undoubtedly due, in part, to a higher incidence of obesity, higher trabecular and/or cortical bone mass, independent of weight, may be a major determinant. The mechanism(s) by which higher bone mass arises in blacks has not previously been investigated. It is possible either (1) that bone mass is higher in blacks at skeletal maturity so that despite similar rates of loss bone mass remains above the fracture threshold; or (2) that trabecular and cortical bone loss is less severe in black women. Currently available cross- sectional studies are inadequate to determine which of these possibilities apply and no previous longitudinal study has compared rates of bone loss in the white and black female populations. Better characterization of the determinants of bone mass and rates of bone loss in these groups could lead to new insights into the etiologies of osteoporosis and to the identification of prevention and treatment modalities. Utilizing single and dual photon absorptiometry to quantitate appendicular and axial bone mass and measurements of hormonal, physical and nutritional parameters known to affect bone metabolism, this study will (1) continue a simultaneous and prospective comparison of bone mass, rates of bone loss and the determinants of skeletal health in 296 white and black women to detect protective and risk factors in the two racial groups; and (2) assess the natural history of bone loss pre-peri-, and postmenopausally to detect the age of onset and rates of clinically significant demineralization. The NIA and the NIH Osteoporosis Consensus Conference of 1984 have called for longitudinal studies of the natural history of skeletal aging, as well as for investigations into minority aging. This proposal responds to both of these priorities. In addition, continuation of this ongoing study, with several years of longitudinal data already gathered, permits a cost effective acquisition of knowledge on the time of onset, duration, and magnitude of clinically significant bone loss in women, without which preventive strategies cannot be formulated.